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IASOS Seminar Series Climate Change 2007: The Physical Science Basis The Working Group I Report of the Intergovernmental Panel on Climate Change Fourth.

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Presentation on theme: "IASOS Seminar Series Climate Change 2007: The Physical Science Basis The Working Group I Report of the Intergovernmental Panel on Climate Change Fourth."— Presentation transcript:

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IASOS Seminar Series Climate Change 2007: The Physical Science Basis The Working Group I Report of the Intergovernmental Panel on Climate Change Fourth Assessment Report Nathan Bindoff and others ACECRC, IASOS, CSIRO MAR University of Tasmania TPAC

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The IPCC is a “remarkable example” of mobilizing expert analysis to inform policymakers Jeffrey Sachs (Nature, 12 August 2004) The IPCC assessments are “dull as dishwater” Tim Flannery, The Weather Makers

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Current atmospheric concentrations of carbon dioxide, methane and nitrous oxide: - exceed pre-industrial values; - have increased markedly since 1750 due to human activities [Revised] Fig. SPM-1 Relatively little variation in concentrations before the industrial era. Rapid rate of increase in concentrations and forcing during the industrial era Carbon dioxide forcing increased by 20% in the last 10 years

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RADIATIVE FORCING (RF) [1750-2005] {Global-average estimates and ranges; typical geographical extent and assessed level of scientific understanding} ANTHROPOGENIC Long-lived greenhouse gases -dominant forcing, with high scientific understanding Other greenhouse gases: ozone  Aerosol Direct forcing: better constrained since TAR  Best estimate for cloud albedo forcing given for first time. Note large and asymmetric uncertainty range.  Land-surface forcings {forcings less than +/- 0.1 Wm -2 not discussed} NATURAL Revised solar forcing less than half of that in TAR - from re-evaluation of the change in the long-term irradiance -Volcanic forcing not shown on figure as it is episodic [Revised] Fig. SPM-2

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Since the TAR, improved understanding and better quantification of the forcing mechanisms  Combined anthropogenic forcing derived for the first time using best estimate and uncertainty of each forcing: not a simple sum of the individual best estimates  Globally-averaged combined anthropogenic radiative forcing since 1750 is positive and causing warming  Combined anthropogenic forcing much larger than that due to solar irradiance change (a natural forcing)

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“Warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global average sea level (see Figure SPM-3).” Direct observations of changes in current climate Global changes in atmospheres, oceans, cryosphere and sea level. Regional changes and changes in extremes Climate variables that are unchanged Paleoclimate support for current changes Much more comprehensive than TAR

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Warming in the Arctic is double that for the globe from 19 th to 21 st century and from late 1960s to present. Warmth 1925 to 1950 in Arctic was not as widespread as recent global warmth. Note different scales Arctic vs Global annual temperature anomalies (°C)

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Smoothed annual anomalies for precipitation (%) over land from 1900 to 2005; other regions are dominated by variability. Land precipitation is changing significantly over broad areas Increases Decreases

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The most important spatial pattern (top) of the monthly Palmer Drought Severity Index (PDSI) for 1900 to 2002. The time series (below) accounts for most of the trend in PDSI. Drought is increasing most places

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f. Magnitude of anthropogenic contributions not assessed. Attribution for these phenomena based on expert judgment rather than formal attribution studies SPM-2. Assessment of human influence

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Projections of Future Changes in Climate Best estimate for low scenario (B1) is 1.8°C (likely range is 1.1°C to 2.9°C), and for high scenario (A1FI) is 4.0°C (likely range is 2.4°C to 6.4°C). Broadly consistent with span quoted for SRES in TAR, but not directly comparable

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Multi-model means of warming for near term (2020-2029) and end of the 21 st century (2090-2099) Likelihood can now be given for the range of global mean warming by an assessment of multiple lines of evidence Higher confidence in projected patterns and regional-scale features Figure SPM-5, TS-28, 10.8, 10.28 Projections of Future Changes in Climate (Chapters 10, 11)

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Multi-model mean of precipitation %-change for a medium SRES scenario (A1B) for 2090-2099 Seasonal precipitation regimes with information on magnitude and inter-model agreement Precipitation increases are very likely in high latitudes in 2090-2099 Decreases are likely in most subtropical land regions in 2090-2099 Figure SPM-6, TS-30, 10.9 Projections of Future Changes in Climate (Chapters 10, 11)

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Very likely that the Atlantic meridional overturning circulation (MOC) will slow down over the course of the 21st century. Very unlikely that the MOC will undergo a large abrupt transition during the 21st century. Longer-term changes in the MOC cannot be assessed with confidence Studies with additional fresh water from melting of the Greenland Ice Sheet suggest that this will not lead to a complete MOC shutdown in the 21 st century. Ch. 10, Fig. 10.15

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Post 2100 changes, Greenland: “…..and that the surface mass balance becomes negative at a global average warming (relative to pre-industrial values) in excess of 1.9 to 4.6°C. If a negative surface mass balance were sustained for millennia, that would lead to virtually complete elimination of the Greenland ice sheet and a resulting contribution to sea level rise of about 7 m.” Almost all marker scenarios exceed 1.9 to 4.6 °C tipping points “.. If radiative forcing were to be stabilized in 2100 at A1B levels11, thermal expansion alone would lead to 0.3 to 0.8 m of sea level rise by 2300 (relative to 1980–1999). “ Implication, while not stated, is that there will be large sea level changes beyond 2100 (eg by 2300 something like 1.5 to 3.5m) Projections of Future Changes in Climate (Chapters 10, 11)

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The IPCC WGI “Headlines” “The balance of evidence suggests a discernible human influence on global climate.” (SAR, 1995) “There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities.” (TAR, 2001) “ Most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations. ” (AR4, 2007) “Discernible human influences now extend to other aspects of climate, including ocean warming, continental-average temperatures, temperature extremes and wind patterns.” (AR4, 2007)

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Projection of sea level rise: Projection of changes in various components of the climate system: Projections of sea level rise consist of contributions from five components. In the AOGCMs, thermal expansion provides the largest contributions to sea level rise in the 21 st century. Current ice sheet models have limitations in representing dynamical processes. Projections provide model-based ranges, and no estimate of their likelihood. Higher values cannot be quantified at this time, but cannot be ruled out. Higher confidence in projected changes of sea ice, snow cover, extreme events, and other regional-scale features. Changes in the Atlantic meridional overturning circulation are now based on multi-model means. Changes beyond the 21 st century cannot be assessed with confidence. Projections of Future Changes in Climate (Chapters 10, 11)

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WGI: Chapter 9 “Understanding and attributing climate change” Number of review comments received: –1438 on First Draft –1157 on Second Draft (~ 50% from one reviewer) All comments are electronically archived (with name of reviewer) Responses to all comments are electronically archived